A non-aqueous electrolyte secondary battery such as a lithium ion secondary battery has been widely used as a power source for mobile device, due to its high energy density, high durability, charge and discharge efficiency, and the like. In recent years, uses as a power source for large-scaled system such as electric motor vehicles such as a hybrid electric vehicle (HEV), an electric vehicle (EV), and a fuel cell vehicle and stationary storage system have been expanded. As a battery used for these power sources, increase in the size of battery, further increase in energy density, high production efficiency for economically achieving these battery performances, and improvement in battery properties accompanying them are required.
The non-aqueous electrolyte secondary battery generally has a configuration in which a positive electrode having a positive electrode active material or the like applied on a current collector and a negative electrode having a negative electrode active material or the like applied on a current collector are connected to each other via an electrolyte layer in which a non-aqueous electrolyte solution or a non-aqueous electrolyte gel is retained within a separator. Charge and discharge reactions of a battery occur by absorption and desorption of ions such as lithium ions on electrode active materials.
One of problems for improving battery properties includes stabilization of the interface between an active material and a non-aqueous electrolyte solution or non-aqueous electrolyte gel. For example, it is known that a solvent, a supporting salt and also an intentionally added additive are decomposed on the electrode surface at an initial charge of lithium ion secondary battery using an electrolyte solution, so that the decomposition product functions as a protective coating film that suppresses a new decomposition of electrolyte (Battery Handbook, edited by The Electrochemical Society of Japan, The Committee of Battery Technology, p. 523 to 546 (2010) (“Battery Handbook”)). This protective coating film is called as solid electrolyte layer (SEI), and is known to greatly contribute to cycle performance, storage performance, charge and discharge efficiency and safety of a battery.
On the other hand, since a non-aqueous electrolyte secondary battery uses a combustible liquid in the electrolyte solution in many cases, a technology for safety improvement is always required, and a battery using an electrolyte that is made into a gel electrolyte so as not to leak is known. U.S. Pat. No. 6,509,123 discloses a battery using a gel electrolyte obtained by forming a copolymer in which hexafluoropropylene is copolymerized with polyvinylidene fluoride (PVdF-HFP) into a gel matrix and swelling it with the electrolyte solution added with vinylene carbonate as an additive. By using such gel electrolyte, chemical stability with the electrode is improved, and the initial charge and discharge efficiency and capacity are improved.
However, when the present inventors have studied, it has been found that, there is a problem in the conventional technology, for forming an electrochemically stabilized electrode active material/electrolyte interface and increasing the energy density of a battery. Specifically, the technology of Battery Handbook is a technology for adding an additive to an electrolyte solution, and proactively forming a decomposition coating film on the interface using an electrochemical reaction, thus a quantity of electricity is necessary for continuing the reaction. Therefore, the coulomb efficiency of the battery is consequently lowered, and becomes the cause of battery capacity reduction.
On the other hand, in the technology described in U.S. Pat. No. 6,509,123, since it goes through a step of infiltrating a gel electrolyte into an electrode active material layer, it is difficult to obtain good adhesion between an active material and the gel electrolyte, thus there is a problem in controlling an electrode active material/electrolyte interface.